Null pointer access: The variable marketcommends can only be null at this location

Bertha 。 2022-08-21 03:20 21阅读 0赞

源代码:

List meunList=backServiceInter.getmeunBypage(9,1);
    		List meunrealList=new ArrayList();
    		Meun meun=null;
    		Photo photo=null;
    		List<Marketmeun> marketlist=new ArrayList<Marketmeun>();
    		marketlist=null;
    		List<Market> marketcommends=new ArrayList<Market>();
    		Market market=null;
    		Marketmeun marketmeun=null;
    		MeunQueryVo meunQueryVo=new MeunQueryVo();
    		for(int i=0;i<meunList.size();i++)
    		{
    			meun=(Meun)meunList.get(i);
    			photo=(Photo)backServiceInter.getPhoto("2",meun.getId());
    			marketlist=backServiceInter.getMarketmeun(meun.getId());
    			marketcommends.clear();
    			/*错误的写法*/  
    			//marketcommends=null;
    			for(int j=0;j<marketlist.size();j++)
    			{
    				marketmeun=(Marketmeun)marketlist.get(j);
    				market=(Market)backServiceInter.findById(Market.class,marketmeun.getMarket().getId());
    				marketcommends.add(j,market);
    				System.out.println("market="+market.getName());
    			}
    			meunQueryVo.setCommendmarkets(marketcommends);
    			//System.out.println("marketcommends="+marketcommends.size());
    			meunQueryVo.setMeun(meun);
    			meunQueryVo.setBigImage(photo.getBigphoto());
    			meunrealList.add(i,meunQueryVo);
    		}

在一般的for循环时，为了避免过多的new 对象，常常采用上面的写法，即把对象先在对象外创建好，然后=null，但是List要注意，

marketcommends=null

意味着把数组置空，下面的

marketcommends.add(j,market);

自然加不进去了，List会默认为空，如果要在每次for循环前清空List，用clear() 即可。